1. Field of the Invention
The present invention relates to a coin testing apparatus which is used in vending machines and the like, and more particularly to a coin testing apparatus which tests the authenticity of deposited coins and determines the denomination of deposited coins.
2. Description of the Prior Art
A conventional coin testing apparatus tests the authenticity and denomination of a coin by determining the size, thickness, weight or material of a coin and comparing the determined data with a preset reference value or reference range.
One such conventional coin testing apparatus determines the authenticity and denomination of a coin by discriminating between the material or size of the coin. A pair of coils are disposed on both sides of a coin path so as to confront each other. An alternating current is applied to one of the coils and an induced voltage is generated in the other coil. When a coin passes through the sensing area between the coils, the induced voltage varies according to the material or size of the coin. The value of the varying induced voltage at a maximum variation point is detected by an electronic circuit. The material or size of the coin, (i.e.--the authenticity and type of the coin) is determined by comparing the detected voltage value with a predetermined coin acceptance range for the type of coins.
In the conventional apparatus, however, when the detected level of the induced voltage varies due to a drift in the electronic circuit caused by deterioration of the coils or the elements constituting the electronic circuit or due to a variation in temperature, the determination of the coins may be in error. In particular, when coins having similar materials or sizes are to be discriminated from each other, the difference between the values of the varying induced voltages detected by the coil is very small. Therefore, if there is a relatively large variation in the detected level of the induced voltage due to the drift in the electronic circuit or due to variation in temperature, a small difference in the detected values of the different types of coins cannot be discriminated. In such a case, the discrimination of the type or the authenticity of coins may be in error.
With respect to the above problem, if a self-tuning coin recognition system such as one disclosed in WO 85/04037 (PCT/US85/00369; JP-A-SHO 61-501349) is used in the apparatus, or if the electronic circuit is constructed using high-durability elements having stable high temperature and high humidity characteristics, the influence due to the above-mentioned variation of the detected voltage levels could be overcome. A self-tuning system requires a complicated statistical operation and whether the device is operating properly cannot be easily confirmed. High-durability elements are expensive and the cost of the apparatus is too high and thus is not practical.